A new ion pair receptor fulfilling a dual function as a chromogenic molecular switch for F− and ratiometric selective recognition of HSO4−

A new ditopic ion pair receptor 1 containing two biindole moieties and a bis-benzocrown ether unit shows a remarkable color switching (ON- and -OFF) function induced by anion (F − ) and cation (K + ) recognition. The ditopic receptor 1 binds in a cooperative fashion to HSO 4 − in the presence of 1 ·K + and acts as a selective ditopic receptor to recognize ion pairs with a wavelength-ratiometric manner.